Wire ropes are used for many purposes such as ropeways, cable cars, ski lifts, chairlifts and elevators, but are of particular importance in the mining industry where they are used for raising and lowering conveyances holding miners, equipment and ore between the mining galleries and the surface. For such applications, wire ropes often have to be of considerable length, e.g. 10,000 feet or more, and must carry considerable loads, including the weight of the ropes themselves in the sections between the conveyances and the mine hoists at the surface used for their deployment.
To ensure operational safety and long operational life of the equipment, the physical condition of such ropes must be monitored frequently and the ropes must be replaced often. An important parameter that indicates rope condition is the so-called “lay length”. Wire ropes are made up of twisted or braided metal wires. Individual metal wires are twisted together to form wire bundles or strands, and then a number of such strands are twisted together to form a rope. The lay length of such a rope is the distance along the rope (measured parallel to the center line or axis of the rope) in which a strand at the surface makes one complete turn or spiral around the rope. Often, the lay length is measured over a few lay lengths (e.g. four) and then the measurement is divided by the number of lay lengths to produce an average lay length value over the measured section. The lay length is known when a rope is first manufactured (or at least after the strands have been allowed to settle into their permanent positions following a few lifting cycles) but it will change during use. In mining applications, the lay length of a rope naturally changes with depth due to the torsional behaviour of stranded hoist rope. These natural variations evolve during the life of the rope and must be monitored to ensure that they remain within established limits. Local variations also occur in the rope at crossover points and can also occur due to localized faults as a result of a number of causes, e.g. corrosion, core deterioration, wire breakage, rope rotation (i.e. strand unlaying), or physical contacts. The relevance of changes of lay length of a rope requires expert interpretation. In general, however, if the lay length of a rope or a section of a rope changes beyond defined limits, or if it changes locally, this may indicate potential failure of the rope, requiring a need for rope replacement.
One known method of measuring lay length is to take the rope out of service and to measure the lay length with direct measuring devices. Naturally, this involves a shutdown of the operating equipment and can therefore be done only infrequently. Furthermore, direct measurement of the lay length of an entire rope is difficult and prone to error because of the lengths of such ropes. On the other hand, measurement of lay length when the rope is in place in lifting apparatus, and especially when the rope is in use, is also difficult, especially because the rope may be covered in grease, dirt and/or provided with a protective cover or sheath made of plastics or other material, thereby obscuring the strands from visual inspection. This is a problem for equipment that relies on the use of photographic imaging or other visual means, such as that disclosed in US published patent application No. 2009/0232383 A1 to Roland Verreet published on Sep. 17, 2009, or published International patent application WO 2011/112734 to Cornelison et al. published on Sep. 15, 2011.
Another approach to monitoring rope condition has relied on the magnetic or electrically-conductive properties of metal ropes which are normally made of electrically-conductive and magnetically-permeable materials. In fact they are almost always made of steel. U.S. Pat. No. 8,058,881 B2 which issued to Engbring et al. on Nov. 15, 2011, discloses a method of measuring a fault in a braided wire by passing an electric current through the braided wire, using a sensor to detect the magnetic field produced by the current, and deducing the location of a fault in the braided wire when the measured magnetic field has a characteristic oscillation corresponding to the lay length or a multiple thereof. However, this device does not measure the lay length of the braided wire, but rather makes an assumption of the lay length to detect a fault.
It is also known to use magnetic properties to measure other rope conditions, e.g. local faults and loss of metallic area of the rope. An example is disclosed in U.S. Pat. No. 5,804,964 which issued to Hamelin et al. on Sep. 8, 1998. This patent discloses a device having a permanent magnet assembly encircling and creating a magnetic flux in the rope, and employing Hall effect sensors and leakage flux sensors to produce a numerical damage index for the rope in real time.
Nevertheless, there is a need for a way of measuring rope lay length, especially when the rope is under tension and in movement in an operating environment.